Ion kinetic energy analysis

ABSTRACT

A method of ion kinetic energy analysis is provided by forming ions including metastable ions from a vaporized sample material, accelerating the ions toward a target collector electrode through an electric field, and for an interval of time for enhancing the probability of decomposition of the metastable ions in an area along a trajectory of the ions intermediate the ion source and the electric field, varying the electric field intensity for causing daughter ions of differing kinetic energy to be successively focused at a point along the trajectory and providing a spectrum display of the intensity of the ions formed at the point in synchronism with the field variation.

United States Patent Major, Jr. 51 June 27, 1972 [54] ION KINETIC ENERGY ANALYSIS Primary Examiner-James W. Lawrence Assistant ExaminerC. E. Church 72 I t H ld W. nven or are Mq or, Jr Trumbull, Conn Attorney Edward R. y Jr- [7 3] Assignee: The Perkin-Elmer Corporation, Norwalk,

Conn. [57] ABSTRACT Filedl y 15, 1970 A method of ion kinetic energy analysis is provided by forming [21] AppL No: 37 565 ions including metastable ions from a vaporized sample material, accelerating the ions toward a target collector electrode through an electric field, and for an interval of time for [52] US. Cl ..250/41.9 ME, 250/419 G enhancing the probability f decomposition f h metastaue [51 Int. Cl i i ..B0ld 59/44 ions in an area along a trajectory f the ions intermediate the Field Of Search G, ion Source and the electric field yi g the electric References Cited tensity for causing daughter ions 0 differing metic energy to FOREIGN PATENTS OR APPLICATIONS be successively focused at a point along the trajectory and providing a spectrum display of the intensity of the ions formed at the point in synchronism with the field variation.

1,149,426 4/1969 Great Britain ..250/41.9 ME

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ION KINETIC ENERGY ANALYSIS This invention relates to analytical instruments. The invention relates more particularly to a spectrometric method and apparatus for charged particle analysis.

Sample materials have been analyzed with mass spectrometric techniques by bombarding a vaporized portion of a sample with electrons in order to form ions. The ions thus created are accelerated through ion deflection fields wherein ions attributable to different sample material components are discriminated. In a double-focusing mass spectrometer for example, ions of increasing mass-charge ratio are successively focused by a scanning magnetic field at an output slit disposed opposite a target collector electrode. A recording or display means is coupled to the collector electrode and provides a visual presentation of mass spectra.

Proper mass focusing necessitates that ions of the same mass enter the magnetic field with substantially the same energy, i.e., velocity. An electric field is established in the doublefocusing mass spectrometer generally intermediate the ion source and magnetic field for providing an energy focal point along the path of the ion trajectory. Ions focused at this intermediate point will exhibit the same kinetic energy upon entering the magnetic sector and ion deflection according to mass will properly occur.

It is known that a small proportion, e.g., 1/1000, of the ions which are created by an ion source exhibit metastable characteristics. A parent or precursor metastable ion m, decomposes into a charged daughter ion mfl and an uncharged particle m of mass (m -m The kinetic energy of an accelerated precursor ion then becomes distributed among these resultant particles in accordance with the mass m and m of the particles. The decomposition of the precursor ion may occur at various points along the trajectory of the ion. The kinetic energy discriminating characteristics of a double-focusing mass spectrometer therefore have an important effect on the transmission of daughter ions through the instrument.

When a decomposition occurs at a location in the instrument between the electric and magnetic sectors of a doublefocusing mass spectrometer, a relatively broad base peak at mass number m is displayed. When the decomposition occurs at a location between the ion source and electric field, the division of the precursorenergy between particles of different masses, m and m results in a daughter ion mf' having less kinetic energy than is necessary for focusing at the electric sector focal point. Accordingly, at a pre-established electric field potential a daughter ion will be defocused and its existence will not be indicated in the mass spectrum.

The analysis of ion decompositions is useful to the analytical chemist since it contributes to an understanding of the molecular structure of the sample material. In my copending U. S. patent application Ser. No. 725,752, filed on May 1, 1968, and which is assigned to the assignee of the present invention, I have described a method and apparatus employing a double-focusing mass spectrometer for determining the existence and identity of daughter and precursor ions. While this technique is advantageous, it requires the initial magnetic focusing of a peak m* and subsequently varying electric sector potential at constant magnetic field intensity in order to achieve focusing of daughter ions at the velocity focal point. These steps must be performed repetitively for each m in a spectrum. A complicated spectrum is therefore laborious to examine and analyze.

It is an object of this invention to provide an improved apparatus and method for analyzing metastable transitions.

Another object of the invention is to provide a spectrum display of daughter ions occurring in a metastable transition.

Another object of the invention is to provide a relatively simple method and means for displaying metastable decompositions.

A further object of the invention is to provide a kinetic energy spectrum analysis method and apparatus.

In accordance with features of the present invention, a method for metastable ion kinetic energy analysis comprises the steps of creating ions, accelerating the ions through an electric field toward a collector electrode and over an interval of time before entry to the electric field which enhances the probability of creation of daughter ions from metastable ions, continuously varying the electric field intensity over a range for causing daughter ions of differing kinetic energy to be successively focused at a point along their trajectory, and providing a spectrum display of the intensity of ions formed at the point in synchronism with the field variation.

Apparatus in accordance with features of the invention comprise an ion source, a target aperture and an ion collecting means disposed opposite the target aperture, means for establishing a scanning electric field for focusing ions of differing kinetic energy at the target aperture, means for accelerating ions over a path along the ion acceleration trajectory intermediate the ion source and field producing means for establishing a high probability of metastable ion decomposition, and means coupled to said ion collecting means for providing a spectrum display of the abundance of ions focused at the target aperture during electric field variations.

A method of metastable ion kinetic energy and mass analy sis in accordance with a feature of this invention is provided with a double-focusing mass spectrometer by varying electric field potential at a constant magnetic field intensity thereby focusing daughter ions of the same energy at an ion collector electrode; displaying a kinetic energy spectrum of abundance of peaks of daughter ions in synchronism with field variations; establishing a field potential at a constant value corresponding to a peak in the kinetic energy spectrum; varying the magnetic field intensity; and providing a mass spectrum display for the value of sector potential.

Both a kinetic energy spectrum and a mass spectrum is therefore available to the analyst for studying the metastable decompositions and the molecular structure of the sample material under analysis.

These and other objects and features of the invention will become apparent with reference to the following specifications and the drawings, wherein:

FIG. 1 is a schematic diagram of an ion kinetic energy analysis apparatus;

FIG. 2 is a schematic diagram of a double-focusing mass spectrometer;

FIG. 3 is an ion kinetic energy spectrum of a sample material; and,

FIG. 4 is an ion kinetic energy spectrum of another sample material.

Referring now to FIG. 1, the ion kinetic energy analysis apparatus constructed in accordance with this invention includes a chamber 10 shown to be generally tubular shaped and which is evacuated to a desired relatively low operating pressure by a vacuum pump means 12. A sample material under analysis is introduced to an ion source 14 of the apparatus from a sample injector 16. This injector is adapted to vaporize liquid or solid sample materials and to introduce a vaporized material to the ion source 14. The ion source 14 includes means for bombarding the vaporized molecules with an electron beam for example in order to form positively or negatively charged ions. These ions are accelerated from the ion source through an apertured entrance plate 18 into the chamber 10. A d.c. accelerating potential is applied to the plate 18 from a source 19. The direction of the accelerated ions is altered by electric sector plates 20 and 22 to which a source of scanning voltage 24 is coupled. The voltage source 24 which provides a ramp type or saw-toothed voltage waveform having a linear scanning segment is applied to the sector plates at terminals 26 and 28 through feedthrough connectors 30 and 32. The source 24 comprises, for example, a potentiometer to which a dc. potential is applied and having a motor driven wiper arm. The scanning voltage thus provided causes ions of successively differing kinetic energy which are deflected by this plate to be focused at an aperture of an aperture plate 34. Ions which are transmitted through the aperture of this plate will strike a collector electrode 36 disposed opposite the aperture plate 34. A

current resulting from the ions striking the collector electrode 36 will generate an electrical signal which is prearnplified and amplified by the amplifier means 38 and is then coupled to a spectrum display means 40. The spectrum display means 40 comprises for example a motor driven chart recorder or oscillographic display. This display means is operated in synchronism with the voltage scanning source 24 by synchronizing the motor drive and provides a spectrum display of ions striking the collector electrode 36. This spectrum will include peaks representative of the abundance of ions for a particular voltage applied to the sector plates and 22.

The probability of decomposition of metastable ions in the space between the ion source and electric sector is enhanced when the drift" time of an ion along this portion of its trajectory is increased. The drift" time is dependent on the length of this path, represented as L in FIG. 1, and the magnitude of accelerating potential provided by the source 19. Probability of decomposition is good when the drift time is greater than 1-10 microseconds. The distance L and the accelerating voltage are selected to provide the desired drift time.

The metastable ion decompositions result in a charged daughter particle m, and an uncharged particle m The electric sector plates 20 and 22 will deflect the charged daughter particles and cause daughter particles of increasing energy to be successively focused at the aperture in the plate 34. The display provided by spectrum display means 40 therefore comprises an energy spectrum of daughter particles resulting from these decompositions. I have found that this spectrum comprises a fingerprint of the material under analysis and therefore provides useful information to the analytical chemist. Thus in addition to a mass spectrum which also provides a fingerprint" of the material under analysis, the ion kinetic energy spectrum provided by my apparatus provides similar but different detailed information regarding the material. Thus, when used either alone or in conjunction with a mass spectrum, it provides useful information to the chemist in analyzing the molecular structure of the sample material.

Metastable ion analysis in accordance with another feature of this invention is performed on a double-focusing mass spectrometer illustrated in FIG. 2. The mass spectrometer shown therein is of the Nier-Johnson type having an electric sector and a magnetic sector. The electric sector includes deflector plates 52 and 54, while the magnetic sector includes an electromagnet 56. An electric scanning potential for application to the plates 52 and 54 is provided by a source 58 and is coupled thereto by a switch 60 and feedthrough terminals 62 and 65. Alternatively an adjustable d.c. potential provided by a source 66 is coupled to these plates through the switch 60 and feedthrough connectors 62 and 64.

A magnetic field is established along the trajectory of ions by the electromagnet 56 and a source of magnetic scanning current 68 which applies a scanning current to the electromagnet. The scanning current is applied to the electromagnet through a switch 70. Alternatively a d.c. current may be applied to the electromagnet from a source 72 via the switch '70. Positively charged ions are formed in an ion source 74 through electron bombardment of a vaporized sample material which is introduced to the ion source from a sample injector 76. These ions are accelerated through an entrance aperture plate 78 and are deflected by the electric sector plates 52 and 54 and focused at a velocity focal point comprising an aperture 79 formed in an externally adjustable plate 80. All or a portion of the ions transmitted through the plate 80 will strike an externally positionable collector electrode 81 depending upon the positioning of this electrode. The velocity focused ions are directed through the magnetic field formed by the magnet 56 and are focused in accordance with their masscharge ratio at an aperture in a target plate 82. The target plate 82 is externally positionable. Those ions transmitted through this aperture will strike a collector electrode 84 which is coupled to a particle multiplier such as an electron multiplier 86. An electrical signal generatedby this multiplier is coupled to an amplifier 88 and then applied to a mass spectrum display means 90 which may comprise a chart recorder or an oscillographic display which is operated in synchronism with the magnet scanning current source 68.

Ions which strike the collector electrode 81 disposed opposite the electric sector aperture plate 80 will generate a signal which is amplified by a preamplifier and amplifying means 92. The amplifying means 92 is coupled to an energy spectrum display means 94. This display means may also comprise a chart recorder or an oscillographic display and is operated in synchronism with the sector scanning voltage source 58 in the same manner as the apparatus of FIG. 1. The accelerated ions follow a trajectory defined by the electric sector plates and electromagnet through a chamber 96 which is shown to be generally tubular shaped for purposes of the illustration and which is evacuated to the desired low pressure by a vacuum pump 98. A typical commercial instrument of the type illustrated in FIG. 2 comprises the I-Iitachi-Perkin-Elmer mass spectrometer Model RMU-7..

Metastable ion analysis is performed with this double-focusing mass spectrometer by positioning the collector electrode 81 opposite the aperture plate 80 so that it collects substantially all of the ions transmitted through the aperture plate. Some of the ions which are accelerated through the aperture in the entrance plate 78 will undergo metastable decomposition in a space between the ion source and electric sector and the scanning voltage source 58 which will deflect the resultant daughter ions will cause ions of successively difi'ering energy to be focused at the aperture 79 in the plate 80. These ions which then strike the collector electrode 81, generate a signal, and form an energy spectrum for the material under analysis. This energy spectrum which as indicated is displayed on a chart or oscilloscope represents an energy fingerprint of the material. The spectrum will include peaks representative of the abundance of ions of different energies. There is a sector potential corresponding to each of these peaks. The analysis is continued by decoupling the scanning voltage source 58 through switch 66 and coupling the adjustable voltage supply 66 to the sector plates. The adjustable dc. voltage source is adjusted to provide a sector potential corresponding to a particular kinetic energy spectrum peak. Scanning current is then applied to the electromagnet 56 and these ions are then mass scanned to provide a mass spectrum for this particular kinetic energy. Thus, both a kinetic energy spectrum and a mass spectrum are provided for a sample material, the mass spectrum being provided for each peak in the kinetic energy spectrum.

The effectiveness of this form of analysis is indicated by FIGS. 3 and 4, which are spectra of Cis and Trans isomers. Under the usual mass analysis techniques, the differences between the Cis and Trans spectra of dimethyl maleate and dimethyl fumarate will hardly be indicated. Thus the operator will not be able to determine from the mass analysis alone which of the materials, i.e.,

exist in the sample. FIG. 3 is the kinetic energy spectrum of dimethyl maleate, while FIG. 4 is the kinetic energy spectrum of dimethyl fumarate, as measured on a I-Iitachi-Perkin-Elmer Model RMU-7 mass spectrometer and as modified in accordance with FIG. 2. The abscissa in each of these figures is a linear display of the total energy supplied by the ion accelerating means. The ordinate represents ion current. When the normal ion current, represented by peak (FIG. 3) and peak 122 (FIG. 4) is provided by an accelerating energy corresponding to a potential V then the spectra peaks are provided for lower potential V in accordance with the relationship:

(V /V (mo/m of energy.

Although these materials would be indistinguishable in a mass spectrum, FIGS. 3 and 4 illustrate clearly distinguishable peaks associated with the materials. More particularly, the peak 110 in the fumarate spectrum of FIG. 4 is absent in the maleate spectrum of FIG. 3, while the peak 112 of the maleate spectrum of FIG. 3 which is simply a small ridge on the leading edge of a peak 114, is represented in the fumarate spectrum as a relatively large and clearly distinguishable peak 116. Thus the kinetic energy spectra of metastable ions clearly distinguishes these particular materials. I have found the same to be true for a large number of other materials and the energy spectra therefore represent a fingerprint" for identifying these different materials.

Thus I have described a method and apparatus for performing ion kinetic energy analysis to provide an energy fingerprint of an unknown sample. I have also described in accordance with features of this invention a method for providing both a kinetic energy spectrum of daughter ions and an associated mass spectrum. In each of these methods and in the apparatus, the instrument operator is advantageously provided with information facilitating the study of the molecular structure of the unknown sample.

While I have illustrated and described a particular embodiment of my invention, it will be understood that various modifications may be made therein without departing from the spirit of the invention and the scope of the appended claims.

I claim:

l. A method of metastable ion kinetic energy and mass analysis comprising the steps of:

forming a plurality of ions including metastable ions from a vaporized sample material;

projecting said ions toward a deflection electric field for a transit time period prior to entry into said deflection electric field that is greater than 1 microsecond to increase the probability of creation of daughter ions from said metastable ions and successively projecting said ions toward a magnetic field;

varying the deflection electric field intensity while maintaining the magnetic field intensity at a constant value for focusing daughter ions resulting from a metastable decomposition at a first collector electrode;

displaying a kinetic energy spectrum of abundance of daughter ions at said first collector electrode in synchronism with electric field intensity variations, said spectrum including at least one peak;

establishing the deflection electric field intensity at a constant value corresponding to a peak in the kinetic energy spectrum;

varying the magnetic field intensity for causing ions of differing masses to be successively focused at a second collector electrode; and

providing a mass spectrum display for the variation in magnetic field intensity corresponding to the selected value of electric field intensity.

IIK 

1. A method of metastable ion kinetic energy and mass analysis comprising the steps of: forming a plurality of ions including metastable ions from a vaporized sample material; projecting said ions toward a deflection electric field for a transit time period prior to entry into said deflection electric field that is greater than 1 microsecond to increase the probability of creation of daughter ions from said meTastable ions and successively projecting said ions toward a magnetic field; varying the deflection electric field intensity while maintaining the magnetic field intensity at a constant value for focusing daughter ions resulting from a metastable decomposition at a first collector electrode; displaying a kinetic energy spectrum of abundance of daughter ions at said first collector electrode in synchronism with electric field intensity variations, said spectrum including at least one peak; establishing the deflection electric field intensity at a constant value corresponding to a peak in the kinetic energy spectrum; varying the magnetic field intensity for causing ions of differing masses to be successively focused at a second collector electrode; and providing a mass spectrum display for the variation in magnetic field intensity corresponding to the selected value of electric field intensity. 